pool: Let HsmStorageHandler2#store accept more than one file

This pushes the batching granularity all the way down to HsmStorageHandler2.

Target: master
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6870/